Mechanical and electromechanical instruments coupled between moving components of vehicles are used to measure the steering angle of one or more components, such as the wheels, relative to the body of the vehicle. In some examples, the instruments include potentiometers, variable resistors and the like to measure corresponding voltage and resistance and associate these measurements with the steering angle of a vehicle component. Each of these instruments requires a mechanical interface between moving components of the vehicle, such as a wheel and a static portion of the body (e.g., the suspension). The mechanical interface is subject to contamination from dirt, water and the like and is also susceptible to violent contact from flying rocks, field debris, work site debris and other vehicle components. Contamination or physical damage to the mechanical interface may interfere with or cause the instrument to fail.
A radio transmitting antenna creates electric and magnetic fields whose amplitudes vary with time, typically alternating polarity in a sinusoidal manner. Very close to the antenna, the fields produced closely resemble time-varying versions of the static fields that would be produced by unchanging antenna voltages or currents. These fields, called “near fields”, are complicated and include both radial and circumferential components. The fields diminish in strength as the cube of the distance from the antenna, if not faster.
The variation in time of near fields creates, in a way that Maxwell's equations describe, another, simpler field structure, that of a radio wave. The radio wave is negligible in strength near the antenna. It diminishes as the square of the distance from the antenna, more slowly than the near field. It dominates any near field at distances that are substantially greater than the wavelength (velocity divided by frequency).
The radio wave comprises the electric and magnetic far fields, whose directions are perpendicular to the wave's direction of travel and perpendicular to each other. The waveforms of the electric and magnetic far fields are the same at any one point in the spherical wave front around the transmitter and are related to each other in strength by the “wave impedance” of whatever medium the wave is traveling through, which for air is virtually that of free space, or about 377 ohms.
Radio direction finding (RDF) has long been used to measure the orientation of airplanes relative to a radio transmitting antenna. RDF uses the far-field radio wave, typically vertically polarized, that is, with the electric field vertical and the magnetic field horizontal, such as that transmitted by a medium-frequency (around 1 megahertz) broadcasting station, whose wavelength is around 300 meters. The airplane is far enough from the transmitter, that is, at least several wavelengths, so that it is in the antenna's far field, that is, in the radiated wave.
The RDF receiver on the airplane operates by finding the direction of the radiated magnetic field. The magnetic field must be in a circumferential direction along the spherical wave front, that is, transverse to the wave's direction of travel, and therefore exactly 90 degrees from the radial direction extending from the airplane to the transmitter. This then reveals the orientation of the airplane relative to the transmitter. RDF thus depends on the radio wave's relatively simple structure and cannot be relied upon to work properly in the near field.